Hadoop備忘:Reduce階段Iterablevalues中的每個值都共享一個對象

/**?

*?Iterate?through?the?values?for?the?current?key,?reusing?the?same?value??

*?object,?which?is?stored?in?the?context.?

*?@return?the?series?of?values?associated?with?the?current?key.?All?of?the??

*?objects?returned?directly?and?indirectly?from?this?method?are?reused.?

*/??

public???

Iterable?getValues()?throws?IOException,?InterruptedException?{??

return?iterable;??

} ?

在Reduce階段，具有相同key的的所有的value都會被組織到一起，形成一種key:values的形式。

一般情況下，我們會針對某個key下的所有的values進(jìn)行處理，這里需要注意一個問題，當(dāng)我們寫下如下代碼的時候：

protected?void?reduce(KEYIN?key,?Iterable?values,?Context?context??

)?throws?IOException,?InterruptedException?{??

for(VALUEIN?value:?values)?{??

context.write((KEYOUT)?key,?(VALUEOUT)?value);??

}??

} ?

我們在一個循環(huán)中，每次得到的value實際上都是指向的同一個對象，只是在每次迭代的時候，將新的值反序列化到這個對象中，以更新此對象的值：

/**?

*?Advance?to?the?next?key/value?pair.?

*/??

@Override??

public?boolean?nextKeyValue()?throws?IOException,?InterruptedException?{??

if?(!hasMore)?{??

key?=?null;??

value?=?null;??

return?false;??

}??

firstValue?=?!nextKeyIsSame;??

DataInputBuffer?nextKey?=?input.getKey();??

currentRawKey.set(nextKey.getData(),?nextKey.getPosition(),???

nextKey.getLength()?-?nextKey.getPosition());??

buffer.reset(currentRawKey.getBytes(),?0,?currentRawKey.getLength());??

key?=?keyDeserializer.deserialize(key);??

DataInputBuffer?nextVal?=?input.getValue();??

buffer.reset(nextVal.getData(),?nextVal.getPosition(),??

nextVal.getLength()?-?nextVal.getPosition());??

value?=?valueDeserializer.deserialize(value); ?

currentKeyLength?=?nextKey.getLength()?-?nextKey.getPosition();??

currentValueLength?=?nextVal.getLength()?-?nextVal.getPosition(); ?

hasMore?=?input.next();??

if?(hasMore)?{??

nextKey?=?input.getKey();??

nextKeyIsSame?=?comparator.compare(currentRawKey.getBytes(),?0,???